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Similar tips to the ones already given, with photos to help you visualize what your portraits might look like. Do you know what the background is likely to be for your outdoor portrait -- color(s), light/dark, etc. Think about that also. The main idea is to accentuate each of those faces, and not have clothing/shoes/socks/hair that detracts from them or takes attention away from them. It is good that you are planning for this a bit ahead of time because it will make a real difference in how the portraits look, and it's probably hard to get everyone together.
As Bill said -- be sure to take multiple shots -- it's hard for one person to "look relaxed and keep their eyes open and hold their position etc.", much less 10 or 12 people to all do it at the same time.